(Invited) Photoelectrode Materials Design for Solar Fuels: Where Are We Now?

Abstract

Solar water splitting offers an attractive solution for renewable energy storage. Ternary metal vanadates have drawn attention for use in photoelectrochemical (PEC) water oxidation due to their low band gap and photostability.1 For example, Cu2V2O7 with a ratio of CuO: V2O5 stoichiometry (2:1) is a visible light active n-type semiconductor with a low band gap (see below), and has seen use in PEC devices because of its high stability for PEC water oxidation.2 This paper describes the development of a hybrid electrochemical/thermal route for the preparation of metal vanadate thin films. Electrodeposition is a facile, low temperature method which can be deployed to prepare thin films of a target semiconductor. Also, the optoelectronic parameters (e.g., photocurrent, flat band potential) of the thin film semiconductors prepared by electrodeposition can be measured easily without any more processing steps. In addition, the thickness of a given thin film semiconductor, a crucial factor for PEC reactions, can be controlled easily in an electrodeposition process.A two-step electrodeposition strategy for ternary oxides3 was deployed in this study. Results will be presented for M-V-O ternary films (e.g., Cu2V2O7) on fluorine doped tin oxide (FTO) substrate. In the first step, a copper thin film was deposited cathodically from copper(II) nitrate in a non-aqueous medium to preclude copper oxide formation. In the second step, the copper thin film was stripped anodically in ammonium metavanadate (NH4VO3) aqueous solution to generate Cu+ and subsequent in situ precipitation with VO3 - species to yield a green CuVO3 film on the substrate. Electrochemical quartz crystal microgravimetry confirmed two- and one-electron stoichiometry for the first and second steps respectively.Finally, aspects related to film stability and charge transfer efficiency as they relates to solar water splitting, will be reviewed in the light of data from our group.